I've been asking about Russian weddings and it seems that someone called тамада is present at every wedding and their job is to entertain the guests, like an animator. I wonder what is expected from such an animator at a wedding? telling jokes, singing, etc? do they earn much money? do people hire them for other occasions, like birthdays?

Тамада is the guy who entertains guests as you correctly mentioned. The person is in charge for whole wedding from start to the end, jokes, tosts, music, quests for guests and etcIn fact you can hire such person for any event, except funeral of course..Every тамада earns different money depending on their "professional skills".